Frozen burgers from processing plants in the UK and Ireland and on shelves at major retail chains like Tesco, were found to contain horse DNA, with some patties containing up to 29 per cent horse meat.

An investigation by British and Irish governments, food authorities and the companies involved is now underway.

British supermarkets have been given until Friday to come up with definitive information on how the blunder could have happened.

The contamination - discovered by Irish food authorities - was slammed by British Prime Minister David Cameron as being " a completely unacceptable state of affairs", The Guardian reported.

While acknowledging the role of supply chains, Cameron said the blame lay primarily with supermarkets.

"It is worth making the point that ultimately retailers have to be responsible for what they sell and where it has come from," he said.

Retailers involved have taken a hit, with Tesco alone plummeting NZD $570 million in value when the news broke on Wednesday.